A typical thermal inkjet has an array of precisely formed nozzles attached to a print head substrate corresponding to an array of firing chambers that receive liquid ink from a reservoir. Each firing chamber may include a thin-film resistor or firing resistor located opposite the nozzle to allow for the presence of ink between the firing resistor and the nozzle. Electric pulses may then be applied to heat the firing resistors to cause a small portion of the ink near the firing resistor to vaporize. The pressure created by this vaporization drives a small amount of ink through the nozzle. The nozzles may be arranged in a matrix array. Properly sequencing the operation of each nozzle in the array causes characters and/or images to form as the print head is moved with respect to a print medium, such as a piece of paper.
Efforts have been made to reduce the cost and size of ink-jet printers and to reduce the cost per printed page. Some of these efforts have focused on developing printers having small, moving print heads that are connected to larger stationary ink reservoirs by flexible ink tubes. This configuration is commonly referred to as “off-axis” printing.
The development of off-axis printing has created the need to precisely control the pressure of the ink at a variety of locations including the ink reservoir and the print head. Print cartridges may have an internal pressure regulator for regulating the flow of ink from an external source into an ink chamber within the print cartridge. Print cartridges with an internal pressure regulator often incorporate a diaphragm in the form of a bag. The inside of the bag is open to the atmosphere. The expansion and contraction of the bag controls the flow of ink into the print cartridge to maintain a relatively constant back pressure at the print head.
However, when too much air has accumulated in the body and/or manifold of the print cartridge, the regulator may no longer have the capacity to maintain negative pressure. At that point, air in the print head may render nonfunctional any pressure regulator internal to, or leading to, the print cartridge. As a result, the desired back pressure may be lost (for example, due to variation in the temperature or pressure of the ambient environment), and ink may drool out of the print head. A drooling print head may cause permanent damage to the printer and will likely be unable to print with an acceptable print quality.
Designs utilizing a separate pressure regulator to address these issues may be relatively complicated. In addition, the use of a separate pressure regulator may limit the operating efficiency of the printing device. Accordingly, recent efforts have been directed to providing a less complicated ink supply system that is able to reliably provide back pressure. Some designs utilize foam placed in the ink supply. As the ink supply is drained, the volume of the ink supply tends to decrease. The foam provides small capillary volumes which retain ink; the capillary attraction of the ink to the capillary volumes creates a back pressure. Similarly, other designs utilize a spring placed in an ink bag. However, with these designs, a significant amount of the ink in the supply may be stranded and therefore wasted. Such waste may require more frequent ink re-supply, thereby increasing the operating cost of the system.